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The AAPG/Datapages Combined Publications Database

South Texas Geological Society Special Publications


Meteor Impact Site, Anacaho Asphalt Deposits: South Texas Field Trip, 1984
Pages 60-98

Geology of the Anacacho Formation in Southwest Texas

Douglas H. Wilson


The Anacacho Formation is an Upper Cretaceous carbonate bank facies located in the Northern Gulf Coastal Plain of southwest Texas. Shallow water carbonate bank sequences of the Anacacho, were formed on and around Late Cretaceous mafic intrusions and extrusive seamounts that penetrated the Austin Chalk seafloor in a narrow zone between the waning Cretaceous Seaway and the young Gulf of Mexico. These carbonate bank sequences shed a large amount of bioclastic debris, and through time, sketetal debris built up sufficient relief into the photic zone, that new bank sequences began to flourish.

Diagenesis of the Anacacho Formation began with the micritization of allochems that lay on the shallow seafloor, followed by subsequent submarine diagenetic events such as the authigenic growth of glauconite and framboydal pyrite nodules. Post-depositional diagenesis resulted in overgrowths and recrystallization. Also, the effects of burial are evident in the form of pressure solutions features and fracturing. When sediments of the Anacacho were later subjected to ground water circulation in the fresh water phreatic zone, dissolution created secondary porosity and resulted in the precipitation of fresh water “Dog-tooth” sparry cement. Hydrocarbons later infiltrated the newly created porosity of the biosparrudites and was devolitalized to asphalt which is mined today in Uvalde County, Texas.

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